This invention relates to methods and apparatus for determining the attitude of a spinning spacecraft or satellite by Doppler measurements made upon transmissions between the satellite and ground station.
It is very desirable to have knowledge of the attitude of the spacecraft when it is in a transfer orbit, so that the thrusters may be pointed in the right direction at the time of firing, to thereby minimize fuel use and eliminate the need to carry excess fuel. More generally, it is desirable to determine the attitude of spin-stabilized spacecraft for knowledge and proper control thereof.
An article entitled "Doppler Measurements as a Source of Attitude Information for the Ulysses Spacecraft", by Massart et al., describes a method for determining the attitude of the spin axis of a spacecraft at a particular location in an interplanetary orbit by, at that location, measuring the Doppler shift imparted by the spin of a transmitting antenna offset from the spin axis to establish one conical locus for the spin axis attitude, and calculating the intersection with a second cone established by a sun sensor. Uncertainty in the antenna offset limits the accuracy.
It is well known that it is extremely important to eliminate excess weight from spacecraft at the time of launch in order to minimize costs, or for the same launch cost to maximize the amount of fuel available for station keeping, and for other reasons. As one example, for a geosynchronous orbit, it is desirable to minimize the amount of equipment which is utilized only for transfer orbits, and which has no function in the final orbit. Such equipment is not only disadvantageous because its weight might be replaced by stationkeeping fuel, but in addition its mass must be moved at each stationkeeping maneuver, thereby consuming additional fuel.